Rule Selection for `:number` and `:integer` is vague.

[eemeli]

At the moment, the Rule Selection section includes this:

message-format-wg/spec/registry.md

Lines 433 to 439 in 47c1062

	If the option `select` is set to `plural`, selection should be based on CLDR plural rule data
	of type `cardinal`. See [charts](https://www.unicode.org/cldr/charts/latest/supplemental/language_plural_rules.html)
	for examples.
	If the option `select` is set to `ordinal`, selection should be based on CLDR plural rule data
	of type `ordinal`. See [charts](https://www.unicode.org/cldr/charts/latest/supplemental/language_plural_rules.html)
	for examples.

We should replace the "should" with a normative "SHOULD", or otherwise clarify the strength of these requirements or recommendations.